Enzymes known as phosphodiesterases (PDEs) function in vivo to hydrolytically cleave the 3′-phosphodiester bond of cyclic nucleotides to thereby form the corresponding 5′-monophosphate. For instance, certain PDEs can hydroylze the 3′-phosphodiester bond of adenosine 3′,5′-cyclic monophosphate (cAMP) so as to form 5′-adenosine monophosphate (5′-AMP), and/or can hydrolyze the 3′-phosphodiester bond of guanosine 3′,5′-cyclic monophosphate (cGMP) so as to form 5′-guanosine monophosphate (5′-GMP). These cyclic nucleotides exert a significant impact on cellular processes by, for example, converting inactive protein kinase enzymes into an active form. The active form of the protein kinase catalyzes various phosphorylation processes that impact on fundamental cellular processes including transcriptional regulation, ion channel function, and signaling protein activity.
Researchers investigating PDEs generally agree that there are at least eleven distinct PDE families, differentiated on the basis of amino acid sequence, substrate specificity and sensitivity to endogenous and exogenous regulators. These families are commonly known as PDE1 through PDE11. In addition, researchers found that cyclic nucleotide concentration is a significant factor in the course of the in vivo inflammatory response. Accordingly, much research has been directed to methods for influencing the concentration of cyclic nucleotides as a means to influence the inflammatory response, and particular attention has been directed at PDE4 activity. One promising area of research is the development of small organic molecules that inhibit PDE activity. By inhibiting PDE activity, these small molecules reduce the amount of cyclic nucleotide that is converted into the (inactive) corresponding 5′-monophosphate, thereby elevating cyclic nucleotide concentration, and indirectly increasing protein kinase activity within the cell.
Many major pharmaceutical companies are working to develop specific small organic molecules into pharmaceutical compositions that function as PDE inhibitors. ROLIPRAM™ (Schering AG) is an example of an early attempt to develop such a composition directed to PDE4. However, while ROLIPRAM™ exhibited marked anti-inflammatory activity, it was also found to demonstrate unwanted side effects including emesis (also known as nausea and vomiting) and potentiation of gastric acid secretion. These undesired side effects caused ROLIPRAM™ to be withdrawn from development as an anti-inflammatory pharmaceutical. An understanding of the cause of these side-effects, and approaches to mitigate them, subsequently became topics of intense study.
It is now recognized that PDE4 exists in two distinct forms, i.e., two conformers. One conformer, known variously as HPDE4 or HARB, is particularly prevalent in the gastrointestinal tract and central nervous system, has a high affinity for ROLIPRAM™ (i.e., has a High Affinity ROLIPRAM™ Binding Site, “HARBS”), and is considered responsible for the unwanted side-effects. The other conformer is known variously as LPDE4 or LARB, is found in immunocompetent cells, and has a low affinity for ROLIPRAM™. Researchers are seeking to develop small molecules that inhibit the catalytic activity of LPDE4 rather than bind to HPDE4, i.e., molecules that have a low LPDE4:HPDE4 ratio where the numerator and denominator are the appropriate IC50 values. In other words, researchers are seeking so-called “second generation” inhibitor molecules that interact with the LPDE4 catalytic site of PDE4, rather than the HPDE4 ROLIPRAM™ binding site, to provide desirable anti-inflammatory effect without unwanted side-effects such as emesis.
The present invention is directed to fulfilling this need in the art, and providing further related advantages as set forth more completely herein.
For additional and more detailed discussion of PDE enzymes, including the history of their discovery, their characterization and classification, their in vivo activity, their inhibition by small organic molecules, and current clinical efforts directed to providing pharmaceutical compositions containing these small molecules, see, e.g., Burnouf, C. et al. “Phosphodiesterase 4 Inhibitors” Annual Reports in Medicinal Chemistry, Vol. 33, Chap. 10, pp 91-109, 1998 (Bristol, J. A., ed.); Essayan, D. M. “Cyclic Nucleotide Phosphodiesterase (PDE) Inhibitors and Immunomodulation” Biochemical Pharmacology 57:965-973, 1999; Souness, J. E. and Foster, M. “Potential of phospodiesterase type IV inhibitors in the treatment of rheumatoid arthritis” Idrugs 1(5):541-553, 1998; Souness, J. E. et al. “Immunosuppressive and anti-inflammatory effect of cAMP phosphodiesterase (PDE) type 4 inhibitors” Immunopharmacology 47: 127-162, 2000; and Torphy, T. J. “Phosphodiesterase Isozymes” Am J. Respir. Crit. Care Med. 157:351-370, 1998, as well as the numerous references cited in these articles.